1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal processing method. More particularly, the invention relates to a signal processing apparatus and a signal processing method for ensuring reliable communication with each of different communicating parties adopting communication methods in such a manner that the time it takes to establish communication with the communicating party of interest is appreciably shortened.
2. Description of the Related Art
Recent years have witnessed the widespread acceptance of traffic control systems, security systems, electronic money systems and others that utilize the noncontact IC card. A typical system using such a noncontact IC card works as follows: when the noncontact IC card comes into the communicable distance of a reader/writer of the system, the reader/writer emits electromagnetic waves to the card via an antenna. In this state, the reader/writer transmits a signal requesting a return of data from the noncontact IC card via the antenna. In response, the noncontact IC card load-modulates the requested data into return data and sends that data to the reader/writer via its antenna section.
The reader/writer receives the load-modulated signal from the noncontact IC card and demodulates the received signal to acquire the return data. This kind of demodulation processing is generally implemented by the demodulation section incorporated in the reader/writer.
The demodulation section is typically structured as a circuit placed in the IC chip. The demodulation section may be incorporated in the reader/writer as mentioned above to demodulate the signal sent from the noncontact IC card. Alternatively, the demodulation section may be included in the noncontact IC card to demodulate the signal coming from the reader/writer.
In the description that follows, the operation mode in which to receive the signal from the reader/writer will be called card mode, and the operation mode in which to receive the signal from the noncontact IC card will be referred to as reader/writer mode. What follows is a description of the demodulation section that operates primarily in card mode.
In this specification, the unit data period representative of one-bit data in the modulated signal will be called an ETU (Elementary Time Unit). It is assumed that the data value of each bit is demodulated according to the bit coding method defined for each communication method in use.
Today, the formats of signals transmitted from the reader/writer to the noncontact IC card or vice versa are defined by standards such as ISO 14443 and ISO 18092. There are three types of signal formats defined by these standards: Type A (ISO 14443-A), Type B (ISO 14443-B), and Type C (also called FeliCa type).
Illustratively, the Type A noncontact IC card generates a return signal by load-modulating a 13.56 MHz carrier using an 847 KHz subcarrier (847.5 KHz, to be more precise) in response to return data to the reader/writer. Specifically, with the Type A format, a data value “1” is expressed when the subcarrier is superposed typically on the first half of one ETU representing one-bit data; a data value “0” is expressed when the subcarrier is superposed on the second half of one ETU.
With the Type A format, the 100% ASK modulation method is adopted per data period. The data to be transmitted is bit-coded using modified Miller coding.
For the Type B format, the 10% ASK modulation method is adopted. This method involves using a carrier signal with its amplitude indicative of a value “1” per ETU and a carrier signal having a 90% amplitude indicative of a value “0.”
The FeliCa format adopts the BPSK modulation method with modulation ratios ranging from 8% to 30% as well as the Manchester coding method. With the FeliCa format, transitions are effected from Low to High levels or vice versa to express a data value “1” or “0” per ETU.
Illustratively, the demodulation section binarizes bit-coded waveforms at a threshold value at intervals of ¼ ETU, thereby identifying a binarized signal sequence with 4 bits per ETU. This process demodulates the data transmitted from the reader/writer.
For example, the threshold value is set to an amplitude ratio of 50% for the Type A format, 95% for the Type B format, and 15% for FeliCa®. The interval in which the waveform is higher than the threshold value is binarized to a value “1” and the interval where the waveform is lower than the threshold value is binarized to a value “0.”
Each of the above-mentioned three formats differently defines information representing a start of communication (frame head) and information denoting an end of communication.
Illustratively, for the Type A format, a predetermined binarized signal sequence coming in as a received signal in a first ETU is interpreted as information representing a start of communication. For the Type A format, a binarized signal sequence coming in with a non-modulation interval following a data value “0” is regarded as information denoting an end of communication.
Illustratively for the Type B format, a binarized signal sequence of SOF (10-bit value “0” and 2-bit value “1”) is defined as a frame head. Also for the Type B format, a binary signal sequence with a data value “1” and a data value “0” for 10 ETU's is defined as information denoting an end of communication.
Illustratively for the FeliCa format, a binarized signal sequence called “Preamble+Sync” appearing at a start of communication is defined as a frame head. “Preamble” is a binarized signal sequence made up of zeros of at least 48 bits, and “Sync” is a binarized signal sequence corresponding to 16 bits representing “B24D” in hexadecimal notation. Also for the FeliCa format, the arrival of a non-modulation interval (amplitude unmodulated in both the first-half (½) ETU and the second-half (½) ETU) is defined as the information denoting an end of communication.
Furthermore, according to ISO 14443 and ISO 18092, there are four bit rates (106 kbps, 212 kbps, 424 kbps, 847 kbps) corresponding to the Type A and Type B formats. There are two bit rates (212 kbps, 424 kbps) corresponding to the FeliCa format.
From now on, the types and communication rates of noncontact IC cards are expected to grow. Under these circumstances, techniques have been proposed to implement a noncontact IC card capable of communicating with reader/writers operating on different communication methods (e.g., see Japanese Patent No. 2003-233787, called the Patent Document 1 hereunder; and Japanese Patent No. 2006-60363, referred to as the Patent Document 2 hereunder).
That is, the proposals present methods and system apparatuses for establishing noncontact communication between the noncontact IC card and the reader/writer each capable of switching between a plurality of communication interfaces, in such a manner that a search is made from one communication method to another until the communication is established.
Illustratively, according to the technique disclosed in the Patent Document 1 cited above, when the inventive setup is to establish the communication method in effect between the noncontact IC card and the reader/writer, a distinction is made between an externally received modulated signal and noise on the basis of the recurrence of the signal. The modulation method is changed successively in the upstream part of the setup for consistency with the coding method that is changed consecutively in the downstream part of the setup. In this manner, a search is made for the communication method compatible with the signal coming from the noncontact IC card or from the reader/writer until communication is established between the two.
One disadvantage of the technique disclosed in the Patent Document 1 is that it takes time to complete the search for the correct communication method. That is because the search is made while the communication method is being changed one after another.
For example, a demodulation selector is disposed on the input side of a plurality of demodulation circuits. The demodulation selector selects one demodulation circuit after another to determine if the selected circuit can demodulate the received signal. If the selected demodulation circuit is found incapable of demodulating the signal, the next demodulation circuit is selected and the determination is again attempted.
If the determination above reveals that the selected demodulation circuit can demodulate the signal, then a coding selector disposed on the output side of the multiple demodulation circuits selects each of a plurality of decoding circuits to determine if the selected decoding circuit can decode the input data. That means the compatible communication method may not be identified during the reception of the first frame. It takes time before the connection is established between the noncontact IC card and the reader/writer and information can be actually exchanged therebetween.
According to the technique disclosed in the Patent Document 2, there is provided a single analog detection section capable of receiving and detecting an input signal from the reader/writer, along with a plurality of demodulation circuits each capable of demodulating the output signal from the analog detection circuit by establishing synchronization with a predetermined signal. Also, there are provided a plurality of detection circuits that can each detect a header from the input value, as well as a plurality of data demodulation circuits each outputting demodulated data resulting from the input value and an enable signal per ETU.
Where the technique disclosed in the Patent Document 2 is in use, the results of header detection output from the multiple detection circuits are examined to determine which of a plurality of predetermined communication methods matches the detected header. With the matching communication method thus determined, one of the results of demodulation by the multiple demodulation circuits is selected according to the selected communication method, and one of the enable signals is selected likewise. The combination of the selected demodulation result with the selected enable signal is then subjected to data processing corresponding to the communication method in effect.
As described, the processes corresponding to a plurality of communication methods are performed in parallel in order to shorten the time required before the connection is established and information can be actually exchanged.
According to the technique of the Patent Document 2, a frame head can be detected simultaneously by a plurality of header detection sections arranged in parallel. This setup makes it possible to identify the communication method compatible with the received signal during the reception of the first frame head.
The end of the frame is determined by matching a frame end signal against a binarized signal per ¼ ETU obtained by binarizing the received signal to a threshold value. The frame end determination can be performed in the same manner as the frame head detection.
It follows that the technique of the Patent Document 2 is faster in determining the applicable communication method than the technique of the Patent Document 1 searching for the communication method in effect by selecting one communication method after another on the part of the reception apparatus until the selected communication method is found to match the received signal. For example, according to the technique of the Patent Document 1, every time the communication method is to be determined, another frame needs to be received. This means that as many frames as the number of communication method candidates may be required to be detected before the matching communication method can be determined. According to the technique of the Patent Document 2, by contrast, a plurality of header detectors are operated in parallel during the reception of a single frame to determine the communication method in use. Thus the communication method determination is completed during the reception of one frame, which appreciably shortens the time required to establish communication.